Mayor Bill de Blasio, opening a new front in his quest to expand workers’ rights, said on Tuesday that he would offer six weeks of fully paid parental leave to 20,000 public employees, making New York City among the most generous municipal family-leave providers in the nation.

New mothers and fathers, including workers who adopt a child or accept one into foster care, would be eligible for the benefits, which far outstrip those currently available to most federal and New York State employees. The mayor plans to enact the plan by executive order, and the benefits would go into effect next year.

Mr. de Blasio, a Democrat who is one of the country’s most liberal urban leaders, has made labor rights a hallmark of his tenure, mandating paid time off for sick workers and urging higher wages. His announcement comes as some Fortune 500 companies, including Microsoft and Netflix, have moved to expand benefits and time off for new parents.

“Too many new parents face an impossible choice: taking care of their child or getting their paycheck,” Mr. de Blasio said in a statement. “This is a common-sense policy that will make for healthier and more financially stable working families.”

The executive order would apply only to nonunionized managers and city workers, a small portion of the city’s 300,000 employees. Among the eligible workers are administrative assistants, budget analysts and accountants.

But leaders of several municipal labor unions praised Mr. de Blasio’s plan and said they hoped to begin negotiations with the administration about extending the benefits to their members. Officials at City Hall said they were eager to begin those talks.

Providing parental leave to the initial group of 20,000 employees will cost $15 million annually. Budget officials said the expense would be completely offset by cutting two vacation days from some longtime employees, who currently receive 27 vacation days a year, and by eliminating a 0.47 percent raise for managers that had been scheduled for 2017.

The cost of extending the benefits to all of the city’s unionized work force was not immediately clear. Any concessions from employees would need to be negotiated with individual unions.

Elected officials at the state and federal level, including President Obama, have called for an expansion of parental leave benefits in recent years, although progress has been slow. In Albany, a bill to guarantee some form of paid family leave for workers passed the State Assembly in March, but did not advance in the State Senate.

Mr. de Blasio has struggled in recent months to find a clear policy victory that he can trumpet to supporters, particularly among his liberal base, from whom he has met resistance to his plan to build more affordable housing. But the announcement of his parental leave plan brought plaudits from notable Democrats and labor leaders alike.

“The mayor’s action puts the city ahead of the curve,” said Representative Carolyn B. Maloney, a Manhattan Democrat, who is sponsoring a bill in Congress to provide six weeks of paid parental leave to federal employees.

Workers’ rights advocates in New York said Mr. de Blasio’s plan, despite applying to a small group of city employees, could aid their own efforts. “It adds to the momentum to pass a broader family leave bill at the state level,” said Nancy Rankin, vice president for policy research at the Community Service Society, a nonprofit advocacy group.

Mr. de Blasio did not hold a public appearance on Tuesday to discuss his executive order. But aides noted that some members of the mayor’s inner circle had firsthand experience with trying to balance a new child with responsibilities at work.

Alicia Glen, the deputy mayor for housing and economic development, recalled having to return to the office three weeks after giving birth to her second daughter while working as a young assistant commissioner at the city’s Department of Housing Preservation and Development.

“Quite frankly, I was pretty exhausted,” Ms. Glen said in a radio interview with 1010 WINS. “I wished that I had had some more time with her when she was little. Knowing that the next generation of employees at the city will have that chance is really a terrific day for all of us.”

A version of this article appears in print on , on Page A21 of the New York edition with the headline: Mayor Offers 6 Weeks of Paid Parental Leave to City’s Nonunionized Workers. Order Reprints | Today’s Paper | Subscribe